Solar DHW drainback system problems

[Solardreamer]

New to this site and hoping I could get some general advise on my Solar DHW system.  Bought a 20 yr old home in Evergreen, CO that has a solar H2O system with 2 plates, 120 gal BWC SolarSaver storage, and 80 gal Rudd Backup H20 heater.  System is controlled with a Novan Differential Control unit, that I am suspecting is on its last legs.  The system appears to be an open drainback system, I say this because there are no visable heat exchangers, and when I unplug the Novan controller/pump assembly I hear a loud rush of water draining down the pipes from the solar collectors.  The problem is that the system does not drainback on it's own, and instead has been running the pumps 24/7 sense I moved in 7 months ago.  I know that this is not what is supposed to happen, but because until recently H20 temps in both heaters was in the 100+ degree range I figured " don't fix if it's not broke".  Well Winter is finally here and this weekend I was alarmed when I noticed the Solar tank was below 50 degrees, and the panel was at 40.  Disconnected system and started it back up the next morning, got the water back into the 100's.  

  I am planning to upgrade the system in the next few months and add some solar radiant and a larger tank, etc.  Can I continue to cycle the system manually until then?  Anyone have any experience with these controllers?  Don't want to shut off completely as my Rudd H2O heater is OLD, and I am afraid if it does not have the preheating it might not make it.  Don't want to replace it as I am planning on using a tankless heater once the new system is installed.  Any help or insight would be appreciated.

Thanks!

Kevin

[crashk6]

Alright, best case scenario your novan is just out of calibration or (and this is what I'd check for first) you've had one of the thermistors go bad, probably the one on the panels.

Now as your working with system with a pressurized potable supply and you just happen to have a SolarSaver tank, I'm going to say more than likely weather the system is drainback or glycol your heat exchanger is inside the storage tank. It's fairly common actually.

It is not uncommon in areas with cold winters for your winter output temps to be significantly lower than summer output, but the fact you can cycle the system manually and get it up to a higher temp really supports the dead thermistor theory (do check it out), but yes you may continue to cycle the system manually as needed to help your backup heater limp along.

But you really should have the system evaluated by someone in person. I or anyone else can give you general advice but without being able to see the system it's quite impossible to spot safety issues or problems that might damage the whole system.

Since you are planning to upgrade the system soon I'm not overly concerned. When you do upgrade be sure your new tankless heater has a temperature sensor for the incoming water. Not all do, infact most just heat the water to a set number of therms above whatever the incoming water temp is. These units are obviously not suitable for a solar preheated systems as you could get scalding water on an erratic basis. This is often an overlooked detail, but there are tankless units designed specifically for pre-heated systems that do a great job.

Hope it helps!

--

crashK6

[jimjjnn]

Could the sensor at the panel be loose or corroded. Sounds like a sensor problem tho it could be the controller.

BTW, new controllers aren't that expesive and come with good install instructions on how to adjust it for your needs.

[jimjjnn]

Besure to get a tankless water heater that is designed for solar preheating. The Takagi TK-1 that I had sensed the incoming and outgoing temps so it never overheated even with 2 showers and dishwasher running at same time. It did lower the shower temp a little when the dishwasher was filling but never went too high after dishwasher was filled. Nice unit and I miss it a lot since I sold the house.

[Solardreamer]

  Well I am not sure when this problem started, but soon after I bought the property and before I moved in I discovered one day that the pressure relief valve for the solar saver ( not on the tank but on the line) was spraying water all over the place, and I think I remember checking the H2O temps and they were really hot.  Shut off the system and called 2 local solar shops, but ended up getting the run around when they found out where I lived and no one ever came out.  I know that ever sense this happened the pumps have been running non stop, not sure if they were before.  I did have to close this valve to keep it from continuing to spray water.  Temp gauge still works on the panel and 2 sensors in the solar H2O heater(labeled top and bottom tank on the control panel).  One strange thing is that the "top" thermostat wire is actually a telephone wire, with 2 wires connecting to the tank and 2 to a thermostat on the line between the 2 storage tanks.  

  The control unit is mounted to a small tank that I just discovered is the heat exchanger(helps to read the writing on the tank!), and it only takes potable H2O.  Sounds like a closed loop drainback system right?  I guess that would explain why there are 2 lines coming into the controller/pump assembly, 2 lines going out to the solar tank, and 2 lines connecting the two tanks.  Still have not figured out this setup, as there are 2 red valves and 2 blue valves connecting to the backup tank and the water lines.  When doing some plumbing recently I shut off 3 of these ( both red and one blue).  I am betting that this has something to do with the recent drop in temperatures.

  Anyone know of a good book or internet site for active solar plumbing?  Thanks for all the help guys, and for the heads up on the tankless heater temp gauge.  Is the Takagi TK1 an electric system?  Unfortunately no natural gas or propane at the home, and as I eventually plan to add a PV system I do not plan to add any either.

Thanks again and Merry Christmas!

Kevin

[jimjjnn]

The Takagi is propane or nat gas. Don't think they make an electric one. I googled for tankless water heaters and did see some electric units but they use a lot of power tho for a short time.